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Physiological cross-sectional area (PCSA), an important biomechanical variable, is an estimate of a muscle’s contractile force potential and is derived from dividing muscle mass by the product of a muscle’s average fascicle length and a theoretical constant representing the density of mammalian skeletal muscle. This density constant is usually taken from experimental studies of small samples of several model taxa using tissues collected predominantly from the lower limbs of adult animals. The generalized application of this constant to broader analyses of mammalian myology assumes that muscle density (1) is consistent across anatomical regions and (2) is unaffected by the aging process. To investigate the validity of these assumptions, we studied muscles of rabbits (Oryctolagus cuniculus) in the largest sample heretofore investigated explicitly for these variables, and we did so from numerous anatomical regions and from three different age-cohorts. Differences in muscle density and histology as a consequence of age and anatomical region were evaluated using Tukey’s HSD tests. Overall, we observed that older individuals tend to have denser muscles than younger individuals. Our findings also demonstrated significant differences in muscle density between anatomic regions within the older cohorts, though none in the youngest cohort. Approximately 50% of the variation in muscle density can be explained histologically by the average muscle fiber area and the average percent fiber area. That is, muscles with larger average fiber areas and a higher proportion of fiber area tend to be denser. Importantly, using the age and region dependent measurements of muscle density that we provide may increase the accuracy of PCSA estimations. Although we found statistically significant differences related to ontogeny and anatomical region, if density cannot be measured directly, the specific values presented herein should be used to improve accuracy. If a single muscle density constant that has been better validated than the ones presented in the previous literature is preferred, then 1.0558 and 1.0502 g/cm³would be reasonable constants to use across all adult and juvenile muscles respectively.

Hapalemur sps. andProlemur simus(bamboo lemurs, collectively) stand out from the relatively homogeneous lemurids because they are bamboo feeders and vertical clingers and leapers. This unique diet presents equally unique challenges, like its verticality, toughness, and toxicity. The bamboo lemurs share the generalized anatomy of the other lemurids, but also display some well‐documented skeletal adaptations, perhaps to overcome the problems presented by their specialization. Soft‐tissue adaptations, however, remain largely unexplored. Explored here are possible soft‐tissue adaptations inHapalemur griseus. We compareH.griseuswith other lemurids,Propithecus,Galago,Tarsier, and a tree shrew. Based on the available anatomical and physiological data, we hypothesize thatHapalemurandProlemurspecies will have differences in hindlimb morphology when compared with other lemurids. We predict thatH.griseuswill have more hindlimb muscle mass and will amplify muscle mass differences with increased type II muscle fibers. Relative hindlimb muscle mass inH.griseusis less than other prosimians sampled, yet relative sural muscle mass is significantly heavier (P< 0.01) inH.griseus. Results show that the soleus muscle ofH.griseushas a higher amount of type II (fast) fibers in plantarflexors. These findings indicate althoughH.griseusshares some generalized lemurid morphology, its diet of bamboo may have pushed this generalized lemurid to an anatomical extreme. We suspect additional bamboo‐specific adaptations in their anatomy and physiology will be uncovered with further examination into the anatomy of the bamboo lemurs. Anat Rec, 2019. © 2019 Wiley Periodicals, Inc. Anat Rec, 303:295–307, 2020. © 2019 American Association for Anatomy